AQA Syllabus focus:
'Biological explanations for schizophrenia, including genetics and inherited vulnerability.'
Genetic explanations argue that schizophrenia partly results from inherited risk. Research shows the disorder runs in families, but modern accounts emphasize multiple genes interacting with vulnerability rather than a single “schizophrenia gene.”
Core idea of the genetic explanation
The genetic explanation proposes that people may inherit a biological predisposition for schizophrenia. This means genes can increase the likelihood of developing the disorder, even though they do not guarantee it. The emphasis is therefore on inherited vulnerability, not direct genetic determination.
Inherited vulnerability is a genetic predisposition that raises the risk of developing schizophrenia without making the disorder inevitable.
This helps explain why schizophrenia is more common among the biological relatives of someone with the disorder. The closer the genetic relationship, the greater the average risk is likely to be.
Polygenic inheritance
Schizophrenia is now understood as polygenic.
Genome-wide association (GWAS) Manhattan plots for schizophrenia, illustrating how genetic risk signals appear as many peaks across different chromosomes rather than a single decisive locus. The comparison over time visually reinforces the polygenic view: as sample sizes increase, more small-effect associations can be detected, consistent with vulnerability being distributed across numerous variants. Source
Polygenic means influenced by many genes, each making a small contribution to risk.
Rather than one abnormal gene causing schizophrenia, researchers think many risk genes combine to increase vulnerability. Some candidate genes appear to affect neurotransmission, brain development, or the efficiency of synaptic functioning. For example, genes such as COMT and DRD2 have been linked to elevated risk, but each contributes only a small effect. COMT is involved in dopamine breakdown, whereas DRD2 affects dopamine receptor functioning, showing how inherited variation may alter susceptibility.
Research also suggests aetiological heterogeneity, meaning that different combinations of genetic factors may produce the same diagnosis.
Aetiological heterogeneity means that the same disorder can have different underlying causes in different people.
This makes schizophrenia especially difficult to explain genetically. One person may have a particular set of risk genes, while another develops the disorder through a different pattern of inherited risk. Large-scale molecular studies, such as those by Ripke et al., have identified many gene locations associated with schizophrenia, supporting the view that vulnerability is spread across numerous genes. This supports a cumulative-risk model in which the total number and combination of risk variants matter more than any one gene.
Evidence from family, twin, and adoption research
Family studies show that schizophrenia occurs more often in the relatives of people with the disorder than in the general population. A commonly cited pattern from Gottesman is that the lifetime risk is about 1% for the general population, around 9% for siblings, about 13% for children with one schizophrenic parent, and much higher when both parents have schizophrenia. This pattern supports a genetic explanation because risk increases with biological relatedness. This dose-response pattern is one of the clearest findings in schizophrenia research.
However, family studies on their own cannot separate genetic influence from shared family environment. Relatives usually live in similar circumstances, so family similarity does not prove that genes alone are responsible.
Twin studies
Twin studies are useful because monozygotic twins share all of their genes, whereas dizygotic twins share about half, like ordinary siblings.
Diagram contrasting monozygotic (identical) and dizygotic (fraternal) twinning, showing how MZ twins originate from one fertilized egg that splits, whereas DZ twins originate from two separately fertilized eggs. This supports the logic of twin studies: MZ pairs share (almost) all their genes while DZ pairs share, on average, about half—so higher MZ concordance is consistent with inherited vulnerability. Source
Researchers compare their concordance rates.
Simplified comparison of monozygotic (MZ) versus dizygotic (DZ) twin pairs, emphasizing the difference in average genetic similarity (about 100% vs about 50%). This helps explain why concordance rates are interpreted as evidence for inherited vulnerability: greater shared DNA in MZ pairs should, on average, produce higher similarity in heritable outcomes. Source
Concordance rate is the extent to which both individuals in a pair share a characteristic or diagnosis.
Twin research is especially valuable because both types of twin usually grow up in the same family, so the main difference between them is their degree of genetic similarity. If schizophrenia has a strong genetic basis, monozygotic twins should show a higher concordance rate than dizygotic twins. Gottesman reported concordance rates of about 48% for monozygotic twins and 17% for dizygotic twins. This difference strongly suggests an inherited component.
At the same time, the concordance rate for monozygotic twins is well below 100%. This shows that genetic factors are important but not sufficient by themselves. If genes were the complete cause, identical twins would always both develop schizophrenia. This is why the explanation is framed in terms of vulnerability rather than certainty.
Adoption studies
Adoption studies offer another way to separate heredity from upbringing. If a child has a biological parent with schizophrenia but is raised by adoptive parents, any increased risk points more directly to genetic influence. Tienari et al. studied adopted children of mothers with schizophrenia and found that these children were more likely to develop the disorder than adopted children without this biological risk. This supports the idea of inherited vulnerability.
Even so, adoption studies have limitations. Adoption is not random, records may be incomplete, and adopted children may still experience unusual early environments before placement. These issues make the evidence strong, but not perfect.
Strengths and limitations of the explanation
A major strength of the genetic explanation is the consistency of findings across different research methods. Family, twin, and adoption studies all show that schizophrenia is more likely when genetic relatedness is higher. This convergence increases confidence that heredity plays an important role.
Another strength is that modern genetics has moved beyond the oversimplified idea of a single schizophrenia gene. The concepts of polygenic inheritance and aetiological heterogeneity fit the evidence better and explain why researchers have found many small-effect genes rather than one decisive mutation.
However, identifying specific genes has been difficult. Many reported associations are weak, and some findings have been hard to replicate. Molecular genetic studies also depend heavily on very large samples, and findings can vary with sample composition. This suggests that genetic risk is complex and may depend on how different genes combine.
A further limitation is that schizophrenia is diagnosed as a broad category. If the diagnosis includes several related but distinct conditions, researchers may be trying to find one genetic explanation for what are actually different disorders. This could reduce the clarity of genetic findings and make the inherited basis seem more confusing than it really is.
Practice Questions
Outline what is meant by inherited vulnerability in genetic explanations of schizophrenia. (2 marks)
1 mark for stating that schizophrenia risk can be inherited or passed on genetically.
1 mark for stating that this creates a predisposition or increased likelihood rather than guaranteeing the disorder.
Discuss genetic explanations for schizophrenia. (6 marks)
AO1 up to 4 marks:
Schizophrenia can involve an inherited predisposition.
Risk is higher in biological relatives.
The disorder is polygenic, involving many genes of small effect.
Credit examples of candidate genes or reference to molecular genetic findings.
Credit family, twin, or adoption evidence, including concordance rates.
AO3 up to 2 marks:
Research support from higher concordance in monozygotic than dizygotic twins.
Concordance is below 100% in monozygotic twins, so genes cannot be the only cause.
Family and twin studies may be confounded by shared environments.
Adoption studies have methodological issues, such as selective placement.
Diagnostic breadth may make gene findings less clear.
FAQ
Candidate gene research starts with a theory about which genes might matter, often based on what is known about brain function.
Genome-wide association studies, or GWAS, do not begin with one favored gene. They scan the whole genome across very large samples to look for patterns linked to schizophrenia.
GWAS is usually seen as less biased because it does not depend as much on prior assumptions, but it needs huge datasets and often finds many very small effects.
Concordance rates can vary because studies use different methods and samples.
Important reasons include:
different diagnostic criteria
different ages at assessment
small sample sizes
whether researchers measure lifetime or current diagnosis
differences in how twins were recruited
So variation between studies does not automatically mean the genetic explanation is weak. It often reflects methodological differences.
Epigenetics refers to changes in gene activity that do not alter the DNA sequence itself.
These changes can be influenced by biological and environmental factors such as:
prenatal stress
infection
nutrition
drug exposure
This is relevant because a person may inherit genetic risk but express that risk differently depending on which genes are switched on or off. Epigenetics helps explain why two people with similar genetic vulnerability can have different outcomes.
Not accurately at the individual level.
Researchers can calculate polygenic risk scores, which estimate how much genetic risk a person carries on average. However, these scores are not precise enough to diagnose schizophrenia or to say that one person definitely will develop it.
That is because:
many genes are involved
each gene has a tiny effect
non-genetic influences still matter
So current genetic testing is more useful for research than for clinical prediction.
Natural selection does not easily remove risk genes when each one has only a very small effect.
Also:
many people with risk variants never develop schizophrenia
some variants may have mixed effects rather than being purely harmful
risk often depends on combinations of genes, not one gene alone
This means schizophrenia-related genetic variants can remain relatively common even if, in some people, they contribute to serious mental illness.
